Magnetic media are widely used in various applications, particularly in the computer industry, and efforts are continually made with the aim of increasing the areal recording density, i.e., bit density of the magnetic media. In this regard, so-called “perpendicular” recording media have been found to be superior to the more conventional “longitudinal” media in achieving very high bit densities. In perpendicular magnetic recording media, residual magnetization is formed in a direction perpendicular to the surface of the magnetic medium, typically a layer of a magnetic material on a suitable substrate. Very high linear recording densities are obtainable by utilizing a “single-pole” magnetic transducer or “head” with such perpendicular magnetic media.
Efficient, high bit density recording utilizing a perpendicular magnetic medium requires interposition of a relatively thick (as compared with the magnetic recording layer), magnetically “soft” underlayer (“SUL”) layer, i.e., a magnetic layer having a relatively low coercivity of about 1 kOe or below, such as of a NiFe alloy (Permalloy), between the non-magnetic substrate, e.g., of glass, aluminum (Al) or an Al-based alloy, and the magnetically “hard” recording layer having relatively high coercivity of several kOe, typically about 3–6 kOe, e.g., of a cobalt-based alloy (e.g., a Co—Cr alloy such as CoCrPtB) having perpendicular anisotropy. The magnetically soft underlayer serves to guide magnetic flux emanating from the head through the hard, perpendicular magnetic recording layer.
A typical conventional perpendicular recording system 10 utilizing a vertically oriented magnetic medium 1 with a relatively thick soft magnetic underlayer, a relatively thin hard magnetic recording layer, and a single-pole head, is illustrated in FIG. 1, wherein reference numerals 2, 2A, 3, 4, and 5, respectively, indicate a non-magnetic substrate, an adhesion layer (optional), a soft magnetic underlayer, at least one non-magnetic interlayer, and at least one perpendicular hard magnetic recording layer. Reference numerals 7 and 8, respectively, indicate the single and auxiliary poles of a single-pole magnetic transducer head 6. The relatively thin interlayer 4 (also referred to as an “intermediate” layer), comprised of one or more layers of non-magnetic materials, serves to (1) prevent magnetic interaction between the soft underlayer 3 and the at least one hard recording layer 5 and (2) promote desired microstructural and magnetic properties of the at least one hard recording layer.
As shown by the arrows in the figure indicating the path of the magnetic flux φ, flux φis seen as emanating from single pole 7 of single-pole magnetic transducer head 6, entering and passing through the at least one vertically oriented, hard magnetic recording layer 5 in the region above single pole 7, entering and travelling along soft magnetic underlayer 3 for a distance, and then exiting therefrom and passing through the at least one perpendicular hard magnetic recording layer 5 in the region above auxiliary pole 8 of single-pole magnetic transducer head 6. The direction of movement of perpendicular magnetic medium 1 past transducer head 6 is indicated in the figure by the arrow above medium 1.
With continued reference to FIG. 1, vertical lines 9 indicate grain boundaries of polycrystalline layers 4 and 5 of the layer stack constituting medium 1. Since magnetically hard main recording layer 5 is epitaxially formed on interlayer 4, the grains of each polycrystalline layer are of substantially the same width (as measured in a horizontal direction) and in vertical registry (i.e., vertically “correlated” or aligned). Completing the layer stack is a protective overcoat layer 11, such as of a diamond-like carbon (DLC), formed over hard magnetic layer 5, and a lubricant topcoat layer 12, such as of a perfluoropolyethylene material, formed over the protective overcoat layer.
Substrate 2 is typically disk-shaped and comprised of a non-magnetic metal or alloy, e.g., Al or an Al-based alloy, such as Al—Mg having an Ni—P plating layer on the deposition surface thereof, or substrate 2 is comprised of a suitable glass, ceramic, glass-ceramic, polymeric material, or a composite or laminate of these materials; optional adhesion layer 2A, if present, may comprise an up to about 30 Å thick layer of a material such as Ti or a Ti alloy; soft magnetic underlayer 3 is typically comprised of an about 500 to about 4,000 Å thick layer of a soft magnetic material selected from the group consisting of Ni, NiFe (Permalloy), Co, CoZr, CoZrCr, CoZrNb, CoFeZrNb, CoFe, Fe, FeN, FeSiAl, FeSiAIN, FeCoB, FeCoC, etc.; interlayer 4 typically comprises an up to about 300 Å thick layer or layers of non-magnetic material(s), such as Ru, TiCr, Ru/CoCr37Pt6, RuCr/CoCrPt, etc.; and the at least one hard magnetic layer 5 is typically comprised of an about 100 to about 250 Å thick layer(s) of Co-based alloy(s) including one or more elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo, Pt, V, Nb, Ge, B, and Pd, iron nitrides or oxides, or a (CoX/Pd or Pt)n, multilayer magnetic superlattice structure, where n is an integer from about 10 to about 25, each of the alternating, thin layers of Co-based magnetic alloy is from about 2 to about 3.5 Å thick, X is an element selected from the group consisting of Cr, Ta, B, Mo, Pt, W, and Fe, and each of the alternating thin, non-magnetic layers of Pd or Pt is up to about 10 Å thick. Each type of hard magnetic recording layer material has perpendicular anisotropy arising from magneto-crystalline anisotropy (1st type) and/or interfacial anisotropy (2nd type).
Another way of classifying perpendicular magnetic recording media into different types is based on the media properties provided by the material(s) utilized for the magnetically hard recording layer(s) 5. For example, as indicated above, the at least one magnetically hard, perpendicular recording layer 5 can comprise magnetic alloys which are typically employed in longitudinal media, e.g., CoCr alloys, or multi-layer magnetic superlattice structures, such as the aforementioned (CoX/Pd or Pt)n, superlattice structures. Referring to FIG. 2 (A), graphically shown therein is an idealized representation of a Kerr hysteresis loop of a perpendicular magnetic recording medium, wherein the nucleation field (Hn), coercivity (Hc), and saturation field (Hsat) are defined. Hn is defined as negative (−) when located in the second (i.e., upper left) quadrant of the graph and as positive (+) when located in the first (i.e., upper right) quadrant. Representative M-H hysteresis loops of magnetic recording layers comprised of these different types of materials are shown in FIGS. 2 (B)–2(C).
As is evident from FIG. 2 (B) showing the M-H loop of a perpendicular recording medium comprising a CoCr alloy, such type media typically exhibit a relatively low coercivity, low remanent squareness, i.e., less than 1, and a positive nucleation field Hn. In addition, the occurrence of magnetic domain reversal within bits, caused by the presence of high demagnetization fields in CoCr-based perpendicular recording media, is problematic with such media in that the phenomenon is a significant source of media noise reducing the SMNR. A high remanent squareness and a negative nucleation field Hn are required in order to obtain good bit stability.
By contrast, and as evidenced by FIG. 2 (C) showing the M-H loop of a perpendicular recording medium comprising a (CoX/Pd)n, multilayer magnetic superlattice structure, such type media advantageously exhibit a relatively high coercivity, remanent squareness of about 1, and a negative nucleation field Hn, which characteristics are attributed to the high anisotropy energy of such type media arising from interfacial anisotropy effects. However, the grains of the multilayer magnetic superlattice structure tend to experience exchange coupling leading to transition noise. Moreover, notwithstanding the possibility of further improvements in multilayer magnetic superlattice structures for use in the fabrication of high recording density magnetic media, significant current issues/problems remain pertaining to the ability to manufacture such structures in a commercially viable manner.
It is believed that high areal recording densities of about 200 Gbit/in2 or greater are possible with perpendicular magnetic media utilizing CoCr-based magnetic alloys as the magnetically hard recording layer. However, the obtainment of such high areal recording densities requires CoCr-based perpendicular media which exhibit the advantageous properties associated with multilayer magnetic superlattice-based media, i.e., high coercivity, remanent squareness of about 1, and a negative nucleation field Hn.
In general, ultra-high areal density perpendicular magnetic recording media require perpendicular magnetic recording layers with high perpendicular anisotropy (Ku) and correspondingly high values of coercivity (Hc) and nucleation field (Hn), which high values are necessary for providing the media with resistance to the large demagnetization effects from the perpendicular recording geometry/system, for maintaining thermal stability with the very small grain sizes (volumes) required for ultra-high areal density recording, and to avoid erasure of the magnetization pattern by the auxiliary pole 8 of the single-pole transducer head 6.
A significant noise source in Co-alloy based perpendicular magnetic recording media is reversed magnetic domains within bits, which reversal is caused by high demagnetization fields within the co-based alloy. As a consequence, high remanent squareness (“St”) and, as explained above, a negative nucleation field (Hn) are required for obtaining good bit stability. The higher values of perpendicular anisotropy (Ku) necessary for obtaining good thermal stability are traditionally obtained by increasing the Pt content of the bulk Co-based magnetic alloy. However, such increase in the Pt content disadvantageously decreases the amount of key segregating elements that can be included in the bulk Co-based alloy and degrades the signal-to-media noise ratio (SMNR) of the media.
In view of the above, there exists a clear need for an improved high areal recording density, magnetic information/data recording, storage, and retrieval media, e.g., perpendicular media, including Co alloy-based magnetically hard recording layers which exhibit substantially increased signal-to-media noise ratios (SMNR), high coercivity, remanent squareness of about 1, and a negative nucleation field Hn. In addition, there exists a need for an improved method for manufacturing high areal recording density, magnetic recording media, e.g., perpendicular media, employing Co alloy-based magnetically hard recording layers, which media exhibit substantially increased signal-to-media noise ratio (SMNR), high coercivity (Hc), remanent squareness (Sr) of about 1, a negative nucleation field (Hn), a narrow switching field distribution (SFD), and can be readily and economically fabricated by means of conventional continuous manufacturing techniques and instrumentalities.
The present invention addresses and solves problems attendant upon the use of Co alloy-based magnetically hard recording layers in the manufacture of high bit density magnetic media such as perpendicular media, which problems include, inter alia, noise generation which adversely affects the SMNR of the media, while maintaining all structural and mechanical aspects of high bit density recording technology. Moreover, the magnetic media of the present invention can be fabricated by means of conventional manufacturing techniques, e.g., sputtering.